Turbo Charging

RW,

That was a terrific "potted" history of Rotax 9 development- my thanks.

The section on the 914 stood out for me.

"In 1993, the Rotax 912 engine was modified and equipped with a turbocharger for an altitude flight test. The airplane (HK36 Super Dimona model) reached an altitude of 33,000 feet (approx. 11,000 m). The concept was successful. The company started the development of the turbocharged Rotax 914 engine with 115 hp, and serial production began in 1996."

It would seem that the desire to achieve high altitude flight, may have been the reason for selecting a TCU, rather than a mechanical wastegate.

I am assuming that a mechanical wastegate would not have the "range " to respond to sea level - 33,000ft changes in atmospheric pressure.

😈

The short history is part of the 100 years book that Rotax produced a few years ago and you can find exerts of it on the www.Rotax.com website.  It is their words not mine, it however gives you some insight as to what thoughts drove them to make the 914.  Remember that one of the motivating factors for them to start to supply aircraft engine derivatives of the Rotax line was to power some Austrian made motor gliders.  The aircraft line of engines started with a modified type 640 (2 stroke, in 1976) that had special ignition and a host of small changes to get it to replace existing engines at the time.  The idea of a flat 4, 4 stroke, came later as pointed out in the article.  The very first prototype 912 flew in a motorglider that is still in the Flight school in Wels Austria where the factory is.  It replaced a modified VW engine.  And as they say, the rest is history.

There is a video of that aircraft on Rotax New Network with an interview of one of the Rotax R&D people.  For many of us in the Rotax family he is considered brilliant and responsible for most of the advancement of the engines for the past 20 years. 

https://www.youtube.com/watch?v=w5THLEBvfc4

Cheers

In this very interesting discussion i'd include the info that another reason to use tcu is to control boost to not exceed maximum pressure ratio of turbo. (2.4 for 914) 

so you can't operate turbocharger over maximum rpm (at the time of 914 design no turbo has rpm speed sensor so pressure ratio was used as a proxy ).

for an airplane operating at altitude with mechanical wastegate is possible to try to recover boost pressure exceeding turbo limit.

moreover tcu can protect from engine  overspeeding, overtemperature of airbox air - limiting boost and permit boost strategies not available with wastegate mech solution

Some people says that in automotive ( mainly in diesel world)  there were a lot of excellent solutions with mech wastegate.it is true

but one thing is a big turbodiesel with low specific output operating on the ground 

Different thing is a carburated small gasoline engine with high specific output operating at variable altitudes with high constant load 

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Some reasons for using tcu were:

1) gasoline no diesel fuel (in diesel detonation it is not a problem - just pump maximum air possible  balancing transient responde and  turbo/engine integrity. so in 912 DETO AVOIDING IT IS A MUST!

2) carburated - you can not use injection parameter to richen miture / use spark timing to avoid detonation

3) altitude. you must control pressure ratio to not exceed turbo limit

4) with tcu you can set a time limit to not exceed engine thermal load at max power (famous 5 minutes) 

5) tcu can implement strategy to richen mixture for avoiding detonation also in carburated engine (3 way solenoid in 914 airbox) 

6) data log . in a certified engine logging it is a must .

in my opinion the 914 tcu is an excellent way of controllIng turbocharging. the cost (argh...) was not a problem. and the control and flexiblity were vastly superior to a mech solution. 

even now the electrical stepper motor actuation of 914 turbo wastegate is up to date (used in supercar/ hypercar engines) in comparison to pneumatic one of 915/916 used in "normal" car